1. The Field of the Invention
Generally, this disclosure relates to firearms. More specifically, the present disclosure relates to methods, devices, and systems for operating a closed-bolt belt-fed firearm with greater reliability of operation, flexibility in platform, and ease of maintenance.
2. Background and Relevant Art
Belt-fed machine guns generally fall into two broad categories based on the way the gun fires ammunition: open-bolt or closed-bolt. In an open-bolt gun, the operating group, which includes the bolt, is held toward the rear of the receiver and away from chamber when not firing. The operating group is restrained, under tension from a spring, such that when the operating group is released, it moves forward forcefully. The forward movement shears a bullet off of a belt, delivers the bullet to the chamber, closes the chamber, and fires the bullet. In a closed-bolt gun, the operating group is held forward and against the barrel extension when not firing. The bolt is mated and locked to the barrel extension forming a closed chamber. The chamber may house a bullet waiting to be fired by an impulse from a hammer or other impulse source delivered to the bullet's primer by a firing pin.
An open-bolt gun is inherently a machine gun. Without input from an operator, an open-bolt gun will continuously fire, typically at a very high rate, as long as the weapon has ammunition or until the gun malfunctions. Each time the operating group moves forward in an open-bolt gun, the forward motion detonates the bullet's primer, firing the gun. The firing of a bullet generates a rapidly expanding gas within the barrel and some of the gas is diverted to a gas piston which forces the operating group rearward, opening the chamber and moving the next round into position, before a spring forces the operating group forward again, repeating the process until the ammunition is exhausted or an operator restrains the operating group in a rearward position.
A closed-bolt gun, conversely, may remain at rest with the operating group forward and a bullet chambered. The firing pin remains withdrawn from the bullet until an impulse source, such as a hammer or a striker, delivers an impulse to the firing pin to detonate the primer and charge in the bullet. At which time, the expanding gas in the barrel may be diverted to provide energy to cycle the operating group similarly to an open-bolt gun, except when the spring returns the operating group to a forward position, the bolt locks adjacent the barrel extension and the bullet in the chamber awaits the operator releasing the impulse source.
Prior to the Firearms Owners' Protection Act of 1986, open-bolt machine guns could be newly registered legally in the United States. The FABRIQUE NATIONALE D'HERSTAL (“FN”) MINIMI open-bolt machine gun (and the affiliated United States variant, the M249 light machine gun platform) was among the most common open-bolt machine guns available at the time, and remains one of the most common open-bolt machine guns in the world. The FN MINIMI was originally developed in 1974 and has continued in operation with militaries in 45 countries. There are a great deal of parts, accessories, and assemblies available for the platform on the market, and the transfer of open-bolt machine guns legally registered before May 19, 1986 is legal through proper channels and with proper documentation. However, the production of new open-bolt machine guns, such as the M249 platform, for civilian sale in the United States is now illegal. Due to the reputation and restricted availability of the M249 platform, there remains a demand for M249-type firearms among civilians, as well as a robust market around the original guns.
However, an open-bolt belt-fed machine gun, such as the M249 platform has a number of disadvantages for use in military or law enforcement conflicts despite the high rate of fire of the weapon. Typically, the high rate of fire of the M249 platform (approximately 800 rounds per minute) results in challenges for the operator to control the recoil and therefore accuracy of the weapon. Furthermore, in many cases, the advantages of outputting up to 800 rounds per minute may be outweighed by the consumption of ammunition. For example, 200 rounds of 5.56 mm×45 mm NATO ammunition, not including the belt links, weighs almost 6 pounds and an M249-platform machine gun can fire all 6 pounds of ammunition in 15 seconds. The M249 platform also supports a 7.62 mm×51 mm NATO variant that weighs twice as much per round. Therefore, mobility of the gun and operator is directly tied to ammunition consumption.
Closed-bolt rifles are legal to manufacture, sell, and own (when properly registered in territories requiring registration) and are not subject to many of the 1986 registration limitations. Closed-bolt rifles capable of full-automatic firing are still regulated. Conversion of a semiautomatic closed-bolt gun to a full-automatic closed-bolt gun is possible with a registered sear that is properly registered with appropriate authorities. However, closed-bolt rifles are capable of semi-automatic fire, burst fire (a fixed number of rounds greater than one), or full-automatic fire with each pull of the trigger. Furthermore, the different firing modes of closed-bolt rifles may be freely selected by a fire mode selector switch commonly mounted on the grip of the rifle allowing a closed-bolt rifle to be freely altered between semi-automatic, burst, and full-automatic firing modes quickly and easily depending on the needs of the operator.
The closed-bolt, hammer- or striker-operated platform, therefore, has operational flexibility that an open-bolt platform cannot offer. Additionally, there are many manufacturers that offer a wide variety of hammer- or striker-operated trigger packages for sale. For example, HECKLER & KOCH manufactures hammer-operated trigger packages that offer selectable fire modes between “safe;” semi-automatic fire; burst fire of two, three, or more rounds at a time; or full-automatic and any combination thereof.
However, an open-bolt gun is not hammer- or striker-operated, and therefore, there is no mechanism by which a hammer or striker may strike a firing pin. Previous attempts to simply drill a bore through the slide and extend the firing pin through the operating group necessitated an additional extension of a hammer beyond the available sizes as is described in “MGA's Semiautomatic MK46 Variant” by Dan Shea, The Small Arms Review, Vol. 13 No. 4, January 2010, pp. 48-54, which is incorporated herein in its entirety by reference. The target operational lifetime for belt-fed firearms is more than 100,000 rounds. The extra length of the bore, firing pin, and hammer all create additional strain on internal components resulting in increased likelihood of firearm failure.
Therefore, it would be desirable to enable the use of a hammer- or striker-operated trigger package with selectable fire modes with an M249-type platform by conversion of the open-bolt M249 or similar platform to a closed-bolt platform and providing a mechanism by which a commercially available standard hammer or striker may impart force to a firing pin.